U–Pb geochronology and REE geochemistry of zircons in mafic granulites from the Lützow-Holm complex, East Antarctica: Implications for the timing and P–T path of post-peak exhumation and Antarctica–Sri Lanka correlation

Abstract

We report new petrological and geochronological data, together with zircon REE patterns, of mafic granulites from the Lützow-Holm Complex (LHC), East Antarctica, and investigate the pressure-temperature-time (P-T-t) evolution of granulite-facies metamorphism in order to unravel the timing and tectonics of Neoproterozoic collisional processes related to the amalgamation of Gondwana. The LHC is composed of three tectonic units: a ca. 2.5 Ga continental fragment (Shirase Microcontinent), a ca. 1.0 Ga magmatic arc (Prince Olav–Vijayan Block; PVB), and a latest Neoproterozoic suture zone (Lützow-Holm Bay Suture Zone; LHSZ) separating the two. Mafic granulites from Tenmondai Rock and Sudare Rock, which belong to the PVB and the LHSZ, respectively, contain orthopyroxene + plagioclase symplectites around garnets, probably formed during post-peak decompression. The application of phase equilibrium modeling and geothermobarometry indicates similar peak P-T ranges of 850–860 °C/7.8–8.4 kbar (Tenmondai Rock) and 800–810 °C/8.5–9.0 kbar (Sudare Rock), together with clockwise P-T paths for both localities. LA-ICP-MS zircon U-Pb dating of the studied granulites yielded Mid-Neoproterozoic magmatic ages of >808 Ma (Tenmondai Rock) and >783 Ma (Sudare Rock) from igneous cores, and Late Neoproterozoic to Cambrian metamorphic ages of ca. 630–481 Ma from overgrown rims and/or structureless grains. Zircon REE analyses indicate that younger 560–510 Ma metamorphic zircons from the localities exhibit heavy-REE-enriched patterns and negative Eu anomalies, suggesting that the consumption of garnet and growth of plagioclase took place during zircon growth. This process was probably related to the formation of post-peak decompressional textures (orthopyroxene + plagioclase symplectite around garnet) observed in the mafic granulites. The results of this study therefore indicate that post-peak exhumation of the LHC started at ca. 560 Ma. Available U-Pb and REE data from 560 to 510 Ma metamorphic zircons of the Highland Complex, Sri Lanka, show similar heavy-REE-enriched patterns and Eu negative anomalies, suggesting that post-peak exhumation of both the LHC and the Highland Complex took place almost simultaneously at ca. 560 Ma. The results of this study further support the proposed tectonic model in which the Highland Complex and the LHSZ are parallel collisional sutures formed during the Late Neoproterozoic-Cambrian Gondwana amalgamation.